This invention relates to an automatic transmission for a vehicle, and more specifically to an automatic transmission arranged to switch gear stages by engagement/disengagement of frictional engagement elements.
An automatic transmission includes planetary gears and a plurality of frictional engagement elements. The automatic transmission is arranged to switch a transmitting path of a rotational torque by engagement/disengagement of the frictional engagement elements, and thereby to attain a plurality of gear stages. Each of the frictional engagement elements includes a piston within a hydraulic chamber. In the engagement of the frictional engagement element, each of the hydraulic chamber receives the hydraulic fluid, so that the corresponding piston is moved. In the disengagement, the hydraulic pressure is released from the hydraulic chamber. In this case, it is desired to perform the engagement/disengagement of the frictional engagement elements in a short time for the switching response (responsiveness). On the other hand, when the engagement and the disengagement are rapidly switched, the rapid variation of the torque is caused, and so that the shock is generated. In particular, at the switching from a running range to a neutral range, the vehicle is switched from a driving (running) state to a state in which the driving force is disappeared. Accordingly, the shock at the disengagement of the frictional engagement elements is notably felt in a relatively silent state in which the driving noise decreases.
Japanese Patent Application Publication No. 2000-352459 discloses an automatic transmission including an orifice switching device provided in a hydraulic control circuit to a frictional engagement element, and arranged to switch between a large orifice state and a small orifice state, and thereby to adjust the variation of the hydraulic pressure. The automatic transmission is arranged to switch the orifice switching device to the large orifice state at the switching from the running range to the neutral range, and thereby to increase the discharge speed of the hydraulic fluid from the frictional engagement element. After the predetermined time period has elapsed, the orifice switching device is switched to the small orifice state to suppress the shock by decreasing the discharge speed. The controller is configured to perform a learning control of the predetermined time period to hold the large orifice state based on a time period from the switching to the large orifice state to a start of generation of the variation of the rotational speed of a predetermined rotating element, and a time period from the switching to the small orifice state to a start of generation of the variation of the rotational speed of the rotating element.